Pit Chains Research Articles

Earliest microbially mediated pyrite oxidation in ~ 3.4 billion-year-old sediments

Pyrite (FeS2) oxidation in modern sedimentary environments is neither a purely chemical nor purely microbial process, but it is significantly enhanced by the activity of microorganisms that use reduced forms of iron and sulphur in their metabolisms. On the early Earth, where oxygen levels were thought to be <10−5 of the present atmospheric level and chemical oxidants scarce, such biological mediation may have been critical in the redox cycles of iron and sulphur. Here, we show that detrital sedimentary pyrite grains in a ~3.4 billion-year-old sandstone were colonised by microbial communities. The detrital pyrite comes from the basal quartz arenite member of the 3.43–3.35Ga Strelley Pool Formation (SPF) in the East Strelley greenstone belt of the Pilbara Craton, Western Australia. Rock chips and petrographic thin sections of black sandstones occurring on two ridges close to the SPF type locality of Strelley Pool were investigated using optical microscopy, SEM, TEM, laser Raman and NanoSIMS. The detrital pyrite grains exhibit laminated carbonaceous coatings of early Archean age, with localised enrichments of nitrogen that are interpreted as the in situ remains of biofilms growing on these nutrient-rich minerals. Pyrite surfaces contain spherical pits, chains of pits and channels that are morphologically distinct from abiotic alteration features. The pits and channels are widespread, have a clustered distribution typical of microbial colonisation, and are closely comparable to biologically mediated microstructures in the younger rock record and those created by extant Fe- and S-oxidising microbes in the laboratory. They are thus interpreted as trace fossils formed by the attachment of bacteria to the pyrite surfaces. A nano-layer and discreet nano-grains of secondary mineral precipitates, namely Fe-oxides belonging to the magnetite-maghaemite group, attest to pyrite oxidation. These are intimately associated with the biofilms and trace fossils, and are interpreted to represent the fossilised mineral products of biologically mediated pyrite oxidation. These data extend the geological range of microbes capable of metabolising reduced Fe and/or S compounds back to the early Archean and indicate that pyrite-rich sedimentary rocks provide promising targets in the search for extraterrestrial life.

Origin of pit chains in the floor of lunar Copernican craters

There are small pit chains in the floor of lunar Copernican craters. They are usually so small in scale that there are few lunar spacecrafts to detect their detailed morphology. Combining camera data from Lunar Orbiter, Lunar Reconnaissance Orbiter (LRO), Kaguya and Chang’e-1 missions, 5 representative large Copernican craters on various terrains of the lunar surface are chosen to study the origin of the pit chains in the crater floor. The morphology and distribution characteristic of the pit chains are referred by the high resolution images in this research. It is suggested that it is the magma activities from the subsurface magma layer combining with the existence of fractures and faults under the crater floor that leaded the formation of the pit chains. The model is further verified and discussed using the regolith thickness data in the crater floor. Our model suggests that the pit chains are still developing in the floor of the Copernican craters and the Moon may not be totally cold. Finally, the model limitation and potential future work are discussed based on available data.

3D seismic imaging of a Tertiary Dyke Swarm in the Southern North Sea, UK

ABSTRACTWe use three‐dimensional (3D) seismic reflection and magnetic data to interpret and describe the 3D geometry of igneous dykes in the southern North Sea. The dykes were emplaced into Paleozoic and Mesozoic sediments and have a common upper termination in Early Tertiary sediments. We interpret the dykes to be part of the British Tertiary volcanic province and estimate the age of the dykes to be 58 Ma. The dykes are characterized by a narrow 0.5–2 km wide vertical disturbance of seismic reflections that have linear plan view geometry. Negative magnetic anomalies directly align with the vertical seismic disturbance zones and indicate the presence of igneous material. Linear coalesced collapse craters are found above the dykes. The collapse craters have been defined and visualized in 3D. Collapse craters have formed above the dyke due to the release of volatiles at the dyke tip and resulting volume loss. Larger craters have potentially formed due to explosive phreatomagmatic interaction between magma and pore water. The collapse craters are a new Earth analogue to Martian pit chain craters.

Interaction of Pipeline Materials with Molten Fluoride Salts

Molten fluoride salts are very promising carriers for the transport of large amounts of heat for example from a high temperature nuclear reactor to a plant which generates hydrogen by chemical processes or from a nuclear reactor to a heat exchanger being a part of the equipment needed to realize the Brayton cycle with a very high power efficiency. Therefore, in the framework of our project, experimental and theoretical investigations of the interactions of fluoride salts as heat carriers needed as high potential and structural materials for pipelines in order to transport heat at temperatures above 600◦C were started. Experimental investigations of Fe-based and Ni-based materials in molten fluoride salts at high temperatures and with different exposure times were performed. Two components salts (LiF-NaF and NaF-NaBF4) and three components salts (LiF-NaF-ZrF4 and LiF-NaF-RbF) were chosen in the experiments. The salt analysis was focussed on the content of metallic elements before and after the exposure of the samples to the salt melts. It was done by inductively coupled plasma-optical emission spectrometry (ICP-OES) and by titrimetric techniques. The thickness of the material zone affected by the salt melts, characterized by an enriched / reduced content of elements in comparison to the mean original content, and the material attacked zone, characterized by very tiny channels or chains of pores or pits formed preferably at grain boundaries, were the subject of the analysis performed by electron microscopy / microprobe techniques. Theoretical models for the transport of elements in the material samples exposed to salt melts using experimental data were also developed.

Deep Sulcal Landmarks Provide an Organizing Framework for Human Cortical Folding

The folding pattern of the cerebral cortex and its relation to functional areas is notoriously variable and there is a need to identify more consistent 3-dimensional (3D) topographical cortical features. We analyzed magnetic resonance brain images of 96 normal adult human volunteers using automated 3D image analysis methods. We examined the deeper parts of the sulci because they generally show less interindividual variability than more superficial parts, especially in monozygotic twins, and deepest parts of primary sulci are the first to develop embryologically and change least as the cortex expands. Along the length of each sulcus we found that there is generally one well-defined zone where depth is maximal, which we term the sulcal pit. Long sulci may have 2 or 3 pits. The spatial arrangement of pits is strikingly regular, forming alternating chains of deeper and shallower pits. We hypothesize that the pits are encoded in the protomap described in Rakic (1988. Specification of cerebral cortical areas. Science. 241:170-176) and are under closer genetic control than the rest of the cortex and are likely to have a more consistent relationship to functional areas.

Control of impact crater fracture systems on subsurface hydrology, ground subsidence, and collapse, Mars

Noachian layered materials are pervasively exposed throughout the highlands of Mars. The layered deposits, in places many kilometers thick, exhibit impact craters of diverse morphologic characteristics, ranging from highly degraded to pristine, most of which formed during the period of heavy bombardment. In addition, exhumed impact craters, ancient channels, and fluvial and alluvial fans are visible in the layered deposits through MOC imagery. These features are more abundant in Noachian terrains, which indicates relatively high erosion rates during ancient Mars that competed with heavy meteoritic bombardment. The Noachian layered materials are thus expected to contain numerous buried impact craters in various states of preservation. Here, we propose that impact craters (buried and exposed) and associated fracture systems dominate the basement structural fabric of the ancient highlands and that they have significantly influenced the hydrogeology. Diversity in the occurrence of high and low densities of impact craters and associated fracture systems controls the magnitude of the local effects of magmatic‐driven hydrothermal activity. In and surrounding the Tharsis region, for example, the formation of chaotic terrains (the source regions of the circum‐Chryse outflow channel system) and a large diversity of collapse structures, including impact crater moats and pit chains, appear to be the result of enhanced hydrothermal activity.

Dilational fault slip and pit chain formation on Mars, David A. Ferrill et al., GSA Today, v. 14, no. 10, p. 4–12.

Dilational fault slip and pit chain formation on Mars, David A. Ferrill et al., GSA Today, v. 14, no. 10, p. 4–12.

Dilational fault slip and pit chain formation on Mars, David A. Ferrill et al., GSA Today, v. 14, no. 10, p. 4–12.

Dilational fault slip and pit chain formation on Mars, David A. Ferrill et al., GSA Today, v. 14, no. 10, p. 4–12.

Distribution, morphology, and origins of Martian pit crater chains

Pit craters are circular to elliptical depressions found in alignments (chains), which in many cases coalesce into linear troughs. They are common on the surface of Mars and similar to features observed on Earth and other terrestrial bodies. Pit craters lack an elevated rim, ejecta deposits, or lava flows that are associated with impact craters or calderas. It is generally agreed that the pits are formed by collapse into a subsurface cavity or explosive eruption. Hypotheses regarding the formation of pit crater chains require development of a substantial subsurface void to accommodate collapse of the overlying material. Suggested mechanisms of formation include: collapsed lava tubes, dike swarms, collapsed magma chamber, substrate dissolution (analogous to terrestrial karst), fissuring beneath loose material, and dilational faulting. The research described here is intended to constrain current interpretations of pit crater chain formation by analyzing their distribution and morphology. The western hemisphere of Mars was systematically mapped using Mars Orbiter Camera (MOC) images to generate ArcView™ Geographic Information System (GIS) coverages. All visible pit crater chains were mapped, including their orientations and associations with other structures. We found that pit chains commonly occur in areas that show regional extension or local fissuring. There is a strong correlation between pit chains and fault‐bounded grabens. Frequently, there are transitions along strike from (1) visible faulting to (2) faults and pits to (3) pits alone. We performed a detailed quantitative analysis of pit crater morphology using MOC narrow angle images, Thermal Emission Imaging System (THEMIS) visual images, and Mars Orbiter Laser Altimeter (MOLA) data. This allowed us to determine a pattern of pit chain evolution and calculate pit depth, slope, and volume. Volumes of approximately 150 pits from five areas were calculated to determine volume size distribution and regional trends. The information collected in the study was then compared with non‐Martian examples of pit chains and physical analog models. We evaluated the various mechanisms for pit chain development based on the data collected and conclude that dilational normal faulting and sub‐vertical fissuring provide the simplest and most comprehensive mechanisms to explain the regional associations, detailed geometry, and progression of pit chain development.

Dilational fault slip and pit chain formation on Mars

Pit crater chains that are parallel or collinear with normal faults are common on Mars, but only rarely observed on Earth. We explore the origin of pit crater chains using recent high-resolution imagery from Mars, laboratory simulation of pit chains, investigation of recent pit chains formed in response to seismic fault slip in Iceland, and assessment of rock failure criteria and the Martian crustal stress field to develop a deeper understanding of the relationship between pit chain formation and fault activity. Based on these analyses, we conclude that pit chains form in response to dilational fault slip. Because gravitational acceleration on Earth (9.81 m/s2) is higher than on Mars (3.72 m/s2), stress within Earth is greater than that of Mars for any given depth. Consequently, steep fault segments are likely to extend to about 5 km depth on Mars compared with about 2 km depth on Earth. Dilation of these steep segments associated with fault slip on Mars could result in large volume increase in the uppermost crust, influencing groundwater flow and discharge and mineralization. Based on crosscutting relationships, pristine pit morphologies, and lack of evidence of sediment accumulation in the bottoms of pits, we interpret that some pit craters may be actively forming in response to dilational fault slip. Figure 1. (A) Composite Viking imagery of normal fault scarps defining horsts and grabens in the southeast part of Alba Patera, Mars. Fault segmentation and en echelon arrangement of faults and grabens are common. Scale bar is 50 km. (B) THEMIS visible spectrum image shows detail of normal faults and pit chain (Phlegethon Catena) shown in A. Scale bar is 5 km. (C) Mars Orbital Camera image shows clear association of pit chains with normal fault scarps within graben on the southeast flank of Alba Patera. Scale bar is 1 km.

Plinian eruptions and passive collapse events as mechanisms of formation for Martian pit chain craters

Three distinct and intersecting sets of graben exist near the Martian volcano Alba Patera. One set formed as a response to the evolution of the Alba Patera edifice. A second set includes mainly linear structures widely interpreted to have formed in response to very large‐scale regional stresses. The members of the third set contain numerous pit craters, and we propose that these craters formed as a result of volcanic activity above long regional dikes emanating from a source to the south within the Tharsis region. The pit craters are of two morphologies: small (&lt;2 km), regular‐shaped ones, usually completely contained within a graben but occasionally unbounded, and larger (4–10 km), irregular ones which commonly coalesce with one another. We argue that these two types of crater formed by different but related mechanisms. The smaller ones developed as the magma in a stalled dike degassed and volatiles were lost to the atmosphere via cracks and fractures in the overlying crustal rocks, which then collapsed into the potential void space at the top of the dike, producing the pit craters. The larger craters formed subsequent to short‐lived explosive eruptions sourced from the underlying dike when the dike walls imploded and collapsed, destroying evidence of any vent structures. There are no visible products of these eruptions because the ambient Martian atmospheric conditions caused the explosive eruptions to be very vigorous, dispersing the products over very wide areas. Evidence indicates that both formation mechanisms occurred within very short distances over the same dike. Preexisting local tectonic stresses appear to have influenced the locations of the pit craters, and groundwater may have been involved in determining their locations and the style of eruptive activity.

The Volcanic and Tectonic History of Enceladus

Enceladus has a protracted history of impact cratering, cryovolcanism, and extensional, compressional, and probable strike–slip faulting. It is unique in having some of the outer Solar System's least and most heavily cratered surfaces. Enceladus' cratering record, tectonic features, and relief elements have been analyzed more comprehensively than done previously. Like few other icy satellites, Enceladus seems to have experienced major lateral lithospheric motions; it may be the only icy satellite with global features indicating probable lithospheric convergence and folding. Ridged plains, 500 km across, consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts. The ridged plains have few if any signs of extension, almost no craters, and an average age of just 107to 108years. Ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells. Our reanalysis of Peter Thomas' (Dermott, S. F., and P. C. Thomas, 1994, The determination of the mass and mean density of Enceladus from its observed shape,Icarus,109, 241–257) limb profiles indicates that high peaks, probably ridge belts, also occur in unmapped areas. Sinuous ridges appear foldlike and are similar to terrestrial fold belts such as the Appalachians. If they are indeed folds, it may require that the ridged plains are mechanically (perhaps volcanically) layered. Regional topography suggests that folding may have occurred along zones of convective downwelling. The cratered plains, in contrast to the ridged plains, are heavily cratered and exhibit extensional structures but no obvious signs of compression. Cratered plains contain a possible strike–slip fault (Isbanir Fossa), along which two pairs of fractures seem to have 15km of right-lateral offset. The oldest cratered plains might date from shortly after the formation of the saturnian system or the impact disruption and reaccretion of Enceladus. Another area of cratered plains has modified craters (e.g., Ali Baba and Aladdin), which some workers have explained by anomalous heat flow and viscous relaxation; lateral shear and shield-building volcanism also may have been important. A young rift-like structure (northern Samarkand Sulci) has few craters and a concentration of cracks or grabens and flattened, flooded, and rifted craters. Pit chains and cratered domes suggest explosive volcanism. Smooth plains may have formed by cryovolcanic equivalents of flood-basalt volcanism. Pure H2O would be difficult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent. Density relations rule out eutectic brine lavas on Enceladus, but NH3–H2O volcanism is possible. Current steady-state tidal dissipation may cause melting of ammonia hydrate at a depth of ∼25 km if the crust is made of ammonia hydrate or ∼100 km if it is made of water ice.

Drainage pits in cohesionless materials: implications for surface of Phobos.

Viking orbiter images show grooves and chains of pits crossing the surface of Phobos, many of which converge toward the large crater Stickney or its antipode. Although it has been proposed that the pits and grooves are chains of secondary craters, their morphology and geometric relations suggest that they are the surface traces of fractures in the underlying solid body of Phobos. Several models have been proposed to explain the pits, of which the most plausible are gas venting and drainage of regolith into open fractures. the latter mechanism is best supported by the image data and is the mechanism studied in this investigation. Drainage pits and fissures are modeled experimentally by using two rigid substrate plates placed edge to edge and covered by uniform thicknesses of dry fragmental debris (simulated regolith). Fracture extension is simulated by drawing the plates apart, allowing drainage of regolith into the newly created void. A typical drainage experiment begins with a shallow depression on the surface of the regolith, above the open fissure. Increased drainage causes local drainage pits to form; continued drainage causes the pits to coalesce, forming a cuspate groove. The resulting experimental patterns of pits and grooves have pronounced similarities to those observed on Phobos. Characteristics such as lack of raised rims, linearity of grooves and chains of pits, uniform spacing of pits, and progression from discrete pits to cuspate grooves are the same in the experiments and on Phobos. In contrast, gas-venting pits occur in irregular chains and have raised rims. These experiments thus indicate that the Phobos grooves and pits formed as drainage structures. The pit spacing in an experiment is measured at the time that the maximum number of pits forms, prior to groove development. The average pit spacing is compared to the regolith thickness for each material. Regression line fits indicate that the average spacing of drainage pits in unconsolidated, noncohesive regolith is nearly equal to the thickness of regolith and appears to gbe independent of the angle of repose, within the resolution of our experiments. This provides a simple means of estimating regolith thickness where drainage pits are present. On Phobos, two locations differing by 90 degrees in longitude have average pit spacings that suggest regolith thicknesses of 290 and 300 m, suggesting that large areas of Phobos have a nearly uniform regolith thickness of approximately 300 m.

The tectonic and volcanic history of Rhea as inferred from studies of scarps, ridges, troughs, and other lineaments

Analyses of landforms on Rhea are used to define 13 types of geomorphic features, interpreted to be of tectonic and volcano-tectonic origin. All types except one seem to be extensional in origin. They occurred after the formation of population I craters and are contemporaneous with a major resurfacing event. The troughs, grabens, grooves, pit chains, scarps, and other lineaments vare “pure” extensional features, while the ridges are volcanic features formed in an extensional stress field. This global surface extension was followed by an era of global compression which produced megaridges and megascarps. These compressional features are not numerous enough to fit the theoretical stresses predicted by the typical thermal models of Rhea. All the extensional landforms appear to form a global grid pattern. This planetary wide grid is similar in direction to the theoretical pattern of a tidally distorted planet. That suggests either an orbital variation very early in the history of Rhea or the existence of an unknown process which has oriented the stress field during the extensional period.

Aerodynamic and vorticity erosion of Mars: Part I. The formation of channels

Wind-erosion processes may include much more than generally realized. Besides abrasion caused by sand-grain impact and saltation, there are several other processes that can be classed as aerodynamic. Three of the most important are vorticity, inter-facial flow, and aerodynamic lift. Using suspended particles as the tools of erosion, the first two of these modes of erosion account for the development of pits, pit chains, and ultimately linear erosion trends. Aerodynamic lift, which occurs in both negative flow and vorticity, is a means of rendering materials airborne in response to differential pressure at extremely low velocity and without need for saltation. Aerodynamic processes provide system to erosion. Hence, shapes of features in most cases define the flow patterns which produced them. This paper is designed to demonstrate how to utilize clues that show aerodynamic influence in the shaping of features, how to make assessments of regional flow systems as related to major topographic features, and how to use the data of aerodynamic and vorticity erosion in assessing the origins of certain features on Mars.

Electron micrography of mineral surfaces subject to wind-blast erosion

The erosional results of two different kinds of wind-blast experiments on isometric minerals, ranging from 2 to 6 in hardness on the Mohs scale, are compared and illustrated, chiefly by electron micrography. One prolonged experiment employed only normal laboratory air. In the other experiment, silt-sized projectiles were added to the air that was blown over one of two similar fluorite specimens for a period of 50 hr during a five-month wind-blast test of the two specimens. In all cases, wind erosion occurred. However, in the second experiment, it was greatly accelerated on the test specimen which was wind-blasted by the air with the added projectiles. Also, these experiments demonstrated in detail where the aerodynamic and vorticity erosion processes concentrate to effect erosion on the shapes that were used. Although wind moves as a fluid, the interfacial flow lines along which erosion occurs form a complex reticulum. Each flow line is attracted to a center of low pressure. Such centers may be within traveling vortices along a flow line but often are at stationary points such as at vortex centers in irregularities along sharp margins where air escapes. Such vortices oriented the flow lines variously so that they merged or even transected one another, despite the use of a single wind direction. Likewise, traveling vortices along flow lines developed pit chains which show all stages of development from isolated scores and pits to fine U-shaped fluting. Thus vorticity plays a major erosive role, and wind can erode in the total absence of sand. Furthermore, aerodynamic erosion can affect every surface of an object, even the basal surface, and may even be greater on the lee face than on the windward face, and the suspended tools produce more definitive erosion than does sand grain impact. In addition, shape and size of the target, its orientation to the wind, inclination of faces, surficial texture and irregularities, and internal constitution are all important determinants of the character, locations, and rates of erosion.

CO 2 permafrost and Martian topography

The role of CO 2 permafrost as an erosive agent on Mars is considered. In the CO 2H 2O system, with a CO 2 triple point at 217°K and 5.1-bar pressure, carbon dioxide solid, liquid, or gas, CO 2 clathrate, and ice are possible stable phases in the range of temperatures and pressures likely to be encountered in the Martian regolith. It is argued that conditions may exist in which CO 2 permafrost is extensive on Mars, provided that adequate CO 2 is available: the maximum ratio of H 2O:CO 2 required in the subsurface pore space system is 17:3. Erosional processes likely to result from such permafrost are block slumping, leading to canyon development; pit chains along faults; chaotic terrain where massive permafrost destruction has occured; large-scale flows of slurry; and perhaps even the flash floods which create channels.

Mars: Troughed terrain

An assemblage of huge parallel troughs, individually up to 200 km wide and hundreds of kilometers long, forms a belt 500 km wide extending for 2700 km E 15°S through the Tithonius Lacus-Coprates region of equatorial Mars. Some troughs are closed depressions more than 3 km deep, and the maximum trough depth may approach 6 km. These troughs, heading just southeast of the huge Tharsis volcanic ridge, are transitional westward into a festoon of U-shaped hollows and eastward into chaotic and fretted terrain. Parallel linear chains of rimless pits and shallow graben on the adjacent cratered upland suggest structural control of trough development by fractures in the Martian crust. Trough walls show scarring by slides, slumps, and U-shaped avalanche chutes. Some are dissected into narrow sharp anastomosing ridges reminiscent of badland topography. Dendritic tributaries extend 150 km into the bordering upland. These tributaries and the mass movements causing recession of trough walls are attributed to a sapping process possibly involving the evaporation or melting of exposed ground ice. Trough floors range from chaotically rough to rolling and subdued. They are not smoothly graded. In one instance layered materials, possibly 2 km thick, compose a dissected trough floor tableland. Most troughs may be partly filled by such deposits. The major problem of trough genesis involves disposal of about 2 × 10^6 km^3 of material. Running water, solution, deflation, and ground ice deterioration all appear to have significant limitations as the sole or principal agent of trough formation. Subsidence caused by magma withdrawal to supply the extensive nearby volcanic fields or the spreading of crustal plates are both quantitatively adequate. Crustal spreading would have significant implications concerning the conditions and the behavior of the Martian interior, and the lack of any obvious subduction zones would imply planetary expansion. In our ignorance concerning Mars both magma withdrawal and crustal spreading merit continued consideration in respect to trough formation, with some favor to the former because of the obvious large-scale Martian volcanism. Ground ice may have played a significant subsidiary role through sapping to produce extensive recession of trough walls.